The Influence of Minor Additions of La and Ce on the Microstructural Components and Forming Properties of Al-1.4Fe Alloys

Abstract

This study investigated the microstructural constituents and forming properties of alloy Al-1.4 wt.% Fe with different additions of Ce and/or La. The addition of rare earth (RE) elements to aluminum alloys improves their microstructures in their as-cast and heat-treated states. RE additions and appropriate heat treatment also improve their mechanical properties. The influence of the homogenization process on the microstructure and forming properties of Al-1.4 wt.% Fe alloy with various additions of Ce and/or La was investigated. When homogenizing the Al-1.4 wt.% Fe alloy at 580 °C, the majority of the homogenization process is completed after 6 h; at 600 °C, after about 5 h; and at 620 °C, after about 4 h. In the micro-alloyed Al-1.4 wt.%–Fe alloy, α-Al, stable Al13Fe4 phases in an agglomerated form, La-containing phases in a spherical form, and Ce-containing phases in a rod-shaped form are present after homogenization. The addition of La was shown to be advantageous as a micro-addition to Al–Fe alloys. Its forming properties show that the combination of Ce and La is the most favorable addition, whereby the homogenization process is fully optimized.

1. Introduction

Recently, rare earth elements have been incorporated into aluminum alloys to manage their microstructures during solidification and enhance their material properties. Lanthanum (La) and Cerium (Ce), which are among the more economically viable rare earths, have been the focus of extensive research [1].

Iron is the primary alloying element in the 8xxx series alloys. Although Fe-rich intermetallic phases are often linked to reduced mechanical properties [2,3], iron is still used in certain wrought aluminum alloys. These alloys are used in rolling due to its favorable balance of ductility and strength at thinner gauges, such as in aluminum foil [4]. The typical Fe content in these alloys ranges from 0.7 to 1.3 wt.%. To enhance workability, homogenization is required before hot rolling an aluminum slab with direct cooling [5]. Since aluminum alloys generally do not solidify under equilibrium conditions, metastable or non-equilibrium intermetallic phases may form.

A high-temperature homogenization treatment is essential to mitigate the effects of segregation, intermetallic phases, and non-equilibrium eutectics [6]. Since homogenization is governed by diffusion, the kinetics of the process are primarily influenced by time and temperature [7]. As the alloy’s temperature increases, atomic mobility is enhanced, leading to the migration of atoms from areas of higher concentration to areas of lower concentration. This movement helps equalize the compositional differences between the centers and edges of the dendrite arms [6]. In the as-cast state, several non-equilibrium or metastable intermetallic phases are present. The only equilibrium phase in the Al–Fe system, present in its aluminum-rich region, is Al₁₃Fe₄ [8]. Due to the low solubility of Fe in the aluminum matrix, only 0.05 wt.% at the eutectic temperature of 655.1 °C [6], most of the Fe in the microstructure forms intermetallic phases, with Al₆Fe, Al_xFe, and Al_mFe being the most common [9,10,11,12,13]. During homogenization, these metastable phases transform into more stable ones [14].

Adding rare earth (RE) elements to aluminum alloys presents an opportunity to enhance their microstructure, corrosion resistance, and mechanical properties. Research has demonstrated the beneficial effects of RE elements on the properties of Al–Fe aluminum alloys. Some studies have also indicated that incorporating Lanthanum (La) and Cerium (Ce) into aluminum alloys can eliminate harmful impurities (the addition of rare earths can reduce the content of impurity solid solutions) [15] such as those of iron (Fe) and silicon (Si), refine grain size, alter their microstructures, and improve both the distribution of inclusion phases and electrical conductivity [16,17,18,19]. In Ce- and/or La-modified Al-1.4Fe alloys, behavior similar to that of RE-free Al-1.4Fe alloys has been observed, with phase transitions occurring at slightly lower temperatures, while the microstructure remains typically hypoeutectic. Al–Fe alloys often contain coarse Al_xFe_y phases with feather-like, plate-like, or needle-like morphologies, which can reduce ductility due to stress concentration [20]. It was reported that the morphologies and distribution of Al–Fe phases can be changed via modification with Ce [21]. The long needle-like primary Al–Fe phase is prominently transformed into fine particles via modification with Ce, either into the claw-like Al–Fe–Ce ternary intermetallic compound and/or the short rod-like Al–Fe–Ce phase (possibly Al₁₃Fe₃Ce phase) [21,22]. It has been reported that adding RE elements leads to grain refinement, with Lanthanum showing the most significant effect [23,24,25]. It was proven that the rare earth adsorbs on the TiB₂ phase surface, reducing its surface energy and size, thereby minimizing the likelihood of agglomeration. It forms a protective shell that prevents movement at the solid/liquid interface, inhibiting the aggregation and growth of the Al₃Ti phase. This leads to a fine, dispersed refining phase that provides more nucleation sites, maintaining effective grain refinement over a long period and improving the anti-fading effect [26,27]. While RE elements do not significantly alter the morphology of the eutectic Al_xFe_y phase, adding an appropriate amount of Ce lowered the recalescence and growth temperatures of the Al–Fe eutectic structure, enhanced the morphology and distribution of the Fe-containing phase, and simultaneously improved the material’s conductivity and mechanical properties [21]. Ce and La participate in the Al–Fe eutectic phase and also form eutectics such as (α-Al + Al₁₁Ce₃) and (α-Al + Al₁₁La₃) [17,21,24,25,28].

A detailed investigation of the influence of rare earths (RE) on solidification and phase formation in Al–Fe alloys was presented in [24]. In this study, additions of Ce and La to A-1.4 wt.% Fe alloy were selected and investigated, as these additions lowered the solidification temperatures of present phases as determined via thermodynamic simulations using Thermo-Calc 2022b software (Thermo-Calc Software AB, Stockholm, Sweden), indicating that the necessary temperature for homogenization may be lower. This study investigated the influence of the homogenization process on the microstructures and the forming properties of Al-1.4 wt.% Fe alloys with different additions of Ce and/or La.

2. Materials and Methods

The addition of rare earth elements (Ce and/or La) to Al-1.4 wt.% Fe alloys was planned and simulated using Thermo-Calc 2022b software and the TCAL6 database. The findings from these simulations are detailed in [24], wherein the experimental alloys were produced using the same methodology. The alloys were melted in an induction furnace, cast into steel molds to form bars with dimensions of 22 mm in diameter and 15 mm in length, and analyzed to confirm their chemical composition. The process involved heating the alloy to 750 °C before adding and mixing the rare earth elements, then casting the mixture into bars. The chemical compositions of the test alloys are provided in

Table 1. Chemical composition in wt.% and designations of experimental alloys.

Sample	Fe	Cu	Si	Mn	Mg	Ce	La	Al
K0	1.36	0.0009	0.0062	0.0013	0.0008	<0.001	<0.001	Rest
K1	1.43	0.0012	0.0081	0.0023	0.0008	<0.001	0.139	Rest
K2	1.38	0.0011	0.0087	0.0058	0.0008	0.147	<0.001	Rest
K3	1.43	0.0012	0.0088	0.0075	0.0009	0.0708	0.0909	Rest

.

Differential scanning calorimetry (DSC) homogenization simulations were conducted using the DSC 404 F1 Pegasus instrument from the NETZSCH Group, Selb, Germany. During these tests, as-cast samples were placed in an empty corundum reference pan, whereby scans were carried out in a dynamic argon atmosphere. The homogenization regime was determined based on the results of the Thermo-Calc simulations presented in [24], where the homogenization temperature was set at 20 °C below the solidus temperature and continued with heating at a rate of 20 K/min to a temperature of 580 °C, 600 °C, or 620 °C and a holding time of 12 h. Cooling was then carried out at a rate of 20 K/min. The tangent method was used to determine the change in the slope of the DSC curves due to changes during homogenization. Based on these results, the homogenization time at the tested temperature was estimated.

The specimens for the microstructural and forming properties tests were cut from the bars in their as-cast state and trimmed to cylinders with a diameter of 10 mm and length of 15 mm. The single-stage solution annealing (homogenization treatment) was carried out in an electric chamber furnace that had previously been calibrated. The test samples were homogenized at 580 °C and 600 °C for 4, 5, 6, 7, 8, 9, 10, 11, and 12 h, followed by quenching in air. Based on the DSC homogenization simulation and the microstructural analysis, samples were selected for further analysis of their forming properties.

All metallographic examinations were performed on as-cast samples and after homogenization annealing at 580 °C and 600 °C for 4, 5, 6, 7, 8, 9, 10, 11, and 12 h, respectively, with the samples ground and polished (no etching) according to the standard metallographic procedure for aluminum alloys. Microscopic examinations were conducted using an Olympus BX61 microscope (Olympus Europa SE and Co. KG, Hamburg, Germany) at 100× and 1000× magnifications. SEM images and EDS maps were captured using a Thermo Fisher Scientific Quattro S FEG SEM equipped with an Oxford Instruments Ultim^® Max EDS SDD analyzer (ThermoFisher Scientific, Waltham, MA, USA).

Forming properties were analyzed using uniaxial hot compression tests. Before performing the hot compression tests, the samples were homogenized at 580 °C and 600 °C for 6 h and 10 h. After homogenization, the samples were cooled in air and machined into cylindrical hot compression samples with a diameter of 10 mm and a length of 15 mm. The Gleeble 3800 thermomechanical simulator with a Hydrawedge II (Dynamic Systems Inc., New York, NY, USA) was used for uniaxial hot compression tests. The cylindrical samples were heated in a vacuum at a rate of 10 K s⁻¹ to the soak temperature of 500 °C and held for 300 s before being hot compressed at a strain rate of 1 s⁻¹ to a true deformation of 1.1. After deformation, the deformed samples were cooled with compressed air to preserve the developed microstructure.

3. Results and Discussion

3.1. Homogenization Simulation

The results of the homogenization simulation performed by DSC measurements are shown in Figure 1. According to the results presented in [24], the highest possible appropriate homogenization temperature was chosen to be 600 °C, but since the standard homogenization temperature for 8xxx series alloys is 580 °C, a simulation of homogenization at 580 °C was also performed. Out of interest, the K0 alloy was also tested at a temperature of 620 °C. The results show that when homogenizing the Al-1.4 wt.% Fe alloy (sample K0) at 580 °C, most of the homogenization process should be completed after 6 h (marked with a green arrow). After this, the chemical composition diffusion process takes its course (the slope of the curve constantly decreases linearly but slightly). In homogenization simulations performed at 600 °C, homogenization should be completed even earlier, namely after about 5 h (marked with a red arrow). The simulations performed at 620 °C indicate that the homogenization process is mostly completed after about 4 h (marked with a black arrow). Since the standard homogenization treatment of 8xxx series alloys is conducted at 580 °C, further investigations were carried out at homogenization temperatures of 580 °C and 600 °C.

3.2. Microstructure Analysis

To confirm the suitability of the homogenization conditions at 580 °C and 600 °C, as predicted by the DSC measurement, the as-cast samples were exposed to 580 °C and 600 °C for 4, 5, 6, 7, 8, 9, 10, 11 and 12 h. All samples were examined under a light microscope, of which only the microstructures of the samples tested at 580 °C and 600 °C for 6 and 10 h are shown in Figure 2. At 500× magnification, sample K0 shows noticeable differences in its intermetallic phase shapes and sizes, with fine spherical and rod-shaped particles likely representing metastable iron-based phases. A darker, more distinct phase, possibly the stable Al₁₃Fe₄ phase, is also observed, as noted in references [29,30]. As the homogenization time increases, the growth of the stable Al₁₃Fe₄ phase in this region becomes evident, while the particles of the phase in the eutectic region grow. After 6 h of homogenization at 580 °C (K0-580 °C/6 h), the proportion of fine spherical particles of the metastable phase decreases significantly but is still visible. In the sample that was homogenized for 10 h, the metastable Al₆Fe phase is no longer visible, as it has completely transformed into a stable Al₁₃Fe₄ phase. This phase also appears in a more agglomerated form. When adding Ce and/or La, the continuous network of eutectic structure Al–Fe phase broke down and to more dispersed and divorced morphology, consistent with [21]. As reported in [29], Ce and La are incorporated into the Al–Fe eutectic, which cannot be seen in the optical micrographs, but the eutectic phases in as-cast state formed by Ce and La are (α-Al + Al₁₁Ce₃) and (α-Al + Al₁₁La₃). These phases are unevenly distributed in the alloy. In all cases, K1, K2, and K3, the completion of the homogenization process cannot be predicted from these micrographs after 6 h but can be predicted after 10 h of homogenization. A similar finding was also made for homogenization at 600 °C (Figure 3), although the proportion of homogenized Fe phases after 6 h is higher than that at a homogenization temperature of 580 °C (Figure 2). It can be said that homogenization is mostly finished after 6 h and is completed after 10 h at 600 °C. The degree of homogenization cannot be specified for the Ce- and La-bearing phases, as they cannot be distinguished in the microstructure.

To be sure that Ce- and La-containing phases were also formed and homogenized, SEM images and EDS maps were made of all the alloys studied, which were homogenized at a temperature of 580 °C for 10 h (Figure 4, Figure 5, Figure 6 and Figure 7). These images confirm the assumptions of the previous analysis regarding the Fe-containing phases, which appear in the matrix in a spherical form and lower density. The Al₁₃Fe₄ phase is thermodynamically more stable than the metastable particles in these conditions. As was reported in [21,22] the presence of Ce in Al-1.4 wt.% Fe alloy can presumably result in the formation of Al₁₀Fe₂Ce compounds. This was also the case in our study, which is shown in Figure 6 and Figure 7, where Ce and Fe appear simultaneously in the mapping. The Ce- and La-containing eutectic phases also agglomerate, with La phases (Figure 5 and Figure 7) forming more spherical shapes, while Ce phases (Figure 6 and Figure 7) typically appear as rods and claws.

3.3. Forming Propetries

The flow curves, which were calculated using the geometry of the cylindrical hot compression samples and the measured force and strain for all combinations of homogenization heat treatments, are shown in Figure 8. The flow curves show the influence of the experimental addition of RE to the Al-1.4Fe alloys on their deformation properties. Alloy K3 exhibited the most favorable forming properties when the samples were homogenized at 580 °C for 6 h (see Figure 8a). The flow stress was slightly lower with a longer homogenization (10 h) at 580 °C, as can be seen in Figure 8b. The additional homogenization time did not contribute to a significant reduction in flow stress. Therefore, a time of 6 h was chosen as the optimum homogenization time at 580 °C. Homogenization at 600 °C led to higher flow stresses during the hot compression tests for both homogenization times and for all alloys with the RE additions (see Figure 8c,d).

Due to the grain refining effect on the α-Al grains and formation of discontinuous eutectic structures of the various Fe intermetallics [25,31] that form in Al–Fe alloys, it was expected that the addition of Ce and La would have a favorable influence on the alloys’ mechanical properties. Uniform homogenization transforms the lamellar metastable and stable Al_xFe_y phases into short rods, which also improves the mechanical properties of the material. Nevertheless, the formation of Al₁₁Ce₃ can reduce the refining effect of the mixed RE combinations. This has a negative effect on the mechanical properties of the alloy, as shown by the agreement of our results with those reported in [1]. The results obtained also show the beneficial use of La as a microadditive in Al–Fe alloys. During the homogenization heat treatment, the La-bearing phases successfully homogenize in the form of small globules (Figure 5 and Figure 7) making the material easier formable due to a content of only small phases and an absence of phases with sharp edges. Ce-bearing phases during the homogenization remain in the form of rods that appear shorter and rounder (Figure 6 and Figure 7), which also improves the formability of the material. Analysis of the forming properties shows that a combined addition of Ce and La is the most favorable RE addition.

4. Conclusions

This paper investigated the influence of minor additions of La or/and Ce on the microstructural components and forming properties of Al-1.4Fe alloys. Samples underwent homogenization treatment under various conditions, whereby the optimum homogenization process was defined, followed by detailed microstructure analyses. The alloy with the best forming properties was then chosen. Our main conclusions are as follows:

When homogenizing the Al-1.4 wt.% Fe alloys at 580 °C, the majority of the homogenization process was completed after 6 h; at 600 °C, after about 5 h; and at 620 °C the homogenization process is mostly completed after about 4 h. After that, only the diffusion process of the chemical composition takes place.

In the as-cast Al-1.4 wt.% Fe samples, very fine spherical particles and rod-like phases are present, representing the metastable Fe-based phases. A stable Al₁₃Fe₄ phase is also present, which is darker and sharper. During homogenization, the stable Al₁₃Fe₄ phase in the interdendritic region grows noticeably. After 6 h at 580 °C, the amount of fine spherical particles from the metastable phase significantly decreases and disappears after 10 h of homogenization at 580 °C; only a stable Al₁₃Fe₄ phase is present in a more agglomerated form. Ce and La integrate into the Al–Fe eutectic, and also form (α-Al + Al₁₁Ce₃) and (α-Al + Al₁₁La₃) eutectic structures. Claw-like Al–Fe–Ce ternary intermetallic compounds and short rod-like Al–Fe–Ce phases (possibly Al₁₃Fe₃Ce phase [21,32]) also form. These phases are unevenly distributed in the alloy; while the completion of the homogenization process cannot be predicted from micrographs after 6 h, it can be predicted after 10 h of homogenization. The proportion of homogenized Fe-phases is higher after 6 h of homogenization at 600 °C than at a homogenization temperature of 580 °C; homogenization is rather complete already after 6 h and certainly complete after 10 h at 600 °C.

After homogenization at 580 °C and 600 °C for 10 h, the Al₁₃Fe₄ phase is thermodynamically more stable than the metastable particles in these conditions. The Ce- and La-bearing phases are also agglomerated and tend to have rounded edges, whereas the La-bearing phases are more spherical than the Ce-bearing phases, which tend to appear in the form of rods and claws.

In the samples homogenized at 580 °C for 6 h, alloy K3 (with 0.15 wt. La and Ce) shows the most favorable forming properties, which even improve slightly with a longer (10 h) homogenization time. A homogenization temperature of 600 °C worsens the forming properties of all RE-containing alloys. Due to the grain refining effect on the α-Al grains and formation of discontinuous eutectic structures of the various Fe intermetallics that form in Al–Fe alloys, it was expected that the addition of Ce and La would have a favorable influence on the alloys’ mechanical properties. Uniform homogenization transforms the lamellar metastable and stable Al_xFe_y phases into short rods. The formation of Al₁₁Ce₃ can reduce the refinement effect of the mixed RE combination, which harms the mechanical properties of the alloy. The addition of La was shown to be advantageous as a micro-addition to Al–Fe alloys. The analysis of the forming properties of these alloys shows that the combination of Ce and La is the most favorable addition.